pinctrl: mvebu: constify mvebu_mpp_ctrl structures

As the mvebu_mpp_ctrl structures contain function pointers, it is
preferable for these to be made read-only to prevent the function
pointers being modified.  So make these const.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>